dubdiscipleIf you have two elements that sound perfect in a vacum on their own and they share a lot of the same frequencies, odds are good they are going to ruin each other in a full mix.
I'd say that was a major breakthrough point for me as well, though I first realised it in a live sound context, long before I had a DAW (actually maybe 10 years before DAWs even existed). It can be difficult to sell that to some musicians though, so sometimes you need one mix for them to hear their instrument solo'd and another one for when it's in a band context. >:-)
There's so many "eureka" moments really, lots of little ones that sometimes come together to make a bigger one...
Realising that guitar tone and volume controls go down as well as up, and that the volume control can do much more than just make things louder or quieter.
How compression works, what it's for, why it's used and when it shouldn't be used.
What you'll get away with in a live performance you won't get away with on a recording that has to stand up to many repeat listenings in a completely different atmosphere.
Sound engineering and musicianing are not the same thing and require different thought processes, even though the jobs overlap.
The recording technology, whether tape and hardware or DAW, is a musical instrument in itself.
Finally, perfecting a recording is a task that is never finished, but at some point you just have to draw a line under it and say "that'll do, release the thing".